The invention relates to a gas distribution system, which can be connected to a gas supply, for admitting and distributing a non-oxidizing gas into a casing which covers at least one solder container and through which printed circuit boards are transported and, during their transport, are brought into contact with a solder wave.
When making flow-soldering systems inert, systems of different design and arrangement are used for the gas distribution. They have the task of making the casing of the solder container inert and hence of adapting the quality of the soldered connections to the increased requirements. With the use of inert gases, in particular nitrogen, the process windows can be adequately enlarged, the wetting can be improved and the formation of scale can be reduced.
In this case, an annular flow, which traverses the transport path of the printed circuit boards to be soldered, is produced via concentrated gas flows that can be directed (DE 42 19 913 A1). In EP 0 500 135 B1 the gas supply is preferably carried out at a limited rate, in order to flow out of the gas distributors in a laminar flow. Since the hood enclosing the solder container is of such a short design that the leading part of a circuit board can come into contact with the solder wave while the trailing part is projecting out of the inlet opening, first and second gas distributors supply non-oxidizing gases, which provide an atmosphere for the underside and the upper side of an incoming circuit board, and associate this with a solder wave. In a manner similar to this, the leading part of a circuit board may project out of the outlet opening while the trailing part is located in the solder wave. Second and third gas distributors therefore accordingly supply non-oxidizing gases. In DE 41 42 436 A1, on the other hand, a diffuser is arranged above the printed circuit boards.
All the current gas distribution systems are based on simple systems of the gas distributors. The inert gas passes via gas supplies to the gas distributor and is fed to the casing via pores (sintered metals or ceramic), according to DE 41 42 436 A1, or tubular openings, according to EP 0 500 135 B1. Gas distributors which have only one chamber with pores, do not have a homogeneous distribution of the inert gas over the pores. Gas distributors having bores or slots at the surface release the inert gas by forming a free jet. This leads to a directed flow. Because of the free-jet principle, the surrounding atmosphere is sucked in. This leads to directed flows and turbulence in the casing and, as a consequence of these directed flows or this turbulence, mixing of the inert gas with oxygen takes place, and the amount of inert gas needed to achieve a required residual oxygen content value becomes correspondingly greater. The gas distributor that is illustrated in EP 0 500 135 B1 comprises a gas supply pipe, a laminar flow being intended to flow out of the gas distributor, in order to suppress the formation of a free jet, as a result of the gas supply at a limited rate. This leads to long flooding times of the casing and to long reaction times in the event of the penetration of oxygen into the casing.
The diffuser made of sintered metal, which is proposed in DE 41 42 436 A1 certainly does not produce a directed flow, but emits the inert gas for this in all directions. In addition, sintered metal elements are susceptible to contamination.
The invention is based on the object of providing a gas distribution system, connected to a gas supply, by means of which large quantities of inert gas can be introduced into the casing, distributed over an area.
According to the invention, this object is achieved in that the gas distribution system has at least one gas distributor arranged above and at least one gas distributor arranged below the printed circuit boards, both being provided with flow means which form a displacement-gas cushion that is distributed homogeneously over the exit areas of the two gas distributors that are directed towards the solder container.
As a result of the fact that the gas distribution system has at least one gas distributor arranged above and at least one gas distributor arranged below the printed circuit boards, both being provided with flow means which form a displacement-gas cushion that is distributed homogeneously over the exit areas that are directed towards the solder container, two displacement-gas cushions are produced in the direction of the solder container and hence of the solder bath. The displacement-gas cushion that is produced by the gas distributor arranged underneath the printed circuit boards experiences a thermal deflection because of the heated solder. The displacement-gas cushion rising from the gas distributor arranged underneath the printed circuit boards is opposed by a displacement-gas cushion emerging from the other gas distributor in the direction of the solder bath, which suppresses any circulation. In this case, the two displacement-gas cushions mix, and a temperature equalization takes place. As a result of the formation of two displacement-gas cushions that emerge from the gas distributors in the direction of the solder bath, residual oxygen contents of less than 1000 ppm are achieved in less than 3 minutes, preferably less than 2 minutes.
As a result of the fact that the gas distributors have a first expansion chamber, which is connected to the gas supply and is connected via at least one opening to at least one further expansion chamber, in which the exit areas for the displacement-gas cushions are provided, the incoming gas flow is expanded in the first chamber and the pressure is reduced. Because of the pressure difference that builds up between the inlet pressure and the first chamber pressure, the inert gas is distributed homogeneously over the entire expansion chamber of the gas distributors. When emerging through the openings of the first expansion chamber, the pressure of the inert gas is reduced further and expanded into the second expansion chamber. The greater volume of the second expansion chamber allows the flow velocity and the pressure to be reduced. Furthermore, the second expansion chamber is used for the quietening and the distribution of the inert-gas flow, preferably of the nitrogen flow.
As a result of the fact that the opening is formed in a direction which is essentially perpendicular to the inlet of the gas supply, further expansion of the inert gas, a reduction in the flow velocity and the conversion of flow energy take place, since one wall of the second expansion chamber is located opposite the openings of the first expansion chamber, with the result that the directed inert-gas flow impinges on this wall, and the flow is thus broken and distributed over the space. By using a plurality of expansion chambers and appropriately arranging a plurality of openings between the first and second expansion chamber, a uniform distribution over the entire length of the gas distributor is advantageously achieved. In the process, any remaining inhomogeneities in the pressure distribution of the first expansion chamber are equalized via the distribution of the number and the size (cross section) of the openings.
A slot-like exit area that is arranged essentially perpendicular to or opposite the flow direction of the openings of the first expansion chamber enables the emergence of the displacement-gas cushion. The gas distributors extend at least over the length of the solder container, the slot-like exit areas being made over the entire length of the gas distributors and producing displacement-gas cushions which, following the emergence from the exit area, correspond to the dimensions of the slot-like exit areas and virtually do not mix with the surrounding atmosphere.
As a result of the fact that the gas distributor arranged under the printed circuit boards surrounds at least the solder wave, a displacement-gas cushion is produced at the point at which the inert gas must primarily be present, the displacement-gas cushion being directly thermally deflected as a result of the arrangement above the heated solder.
In the case of the gas distributor that is arranged under the printed circuit boards and surrounds at least the solder wave, the first expansion chamber is arranged with a spacing in the second expansion chamber, and the opening or the openings of the first expansion chamber open into the second expansion chamber in the direction of the printed circuit boards. One wall of the second expansion chamber is therefore opposite the openings, with the result that the directed and concentrated inert-gas flow impinges on this wall, and the flow is thus broken and distributed over the space. The openings vary in number and size (cross section) over the length of the gas distributor in such a way that a homogeneous gas distribution in the second expansion chamber is achieved. The inert gas, which is expanded and distributed over the U-shaped second expansion chamber, can now leave the gas distributor at the underside, opposite the opening, via slot-like exit areas arranged on both sides in relation to the first expansion chamber. The slot-like exit areas are not narrowed in relation to the dimensions of the second expansion chamber. The U-shaped second expansion chamber is bounded on the open side by the first expansion chamber, the interspace forming the second expansion chamber.
Advantageous developments are specified in the subclaims.